Study of Comet Particles Reveals Surprises Good and Bad

Working in part at NSLS, a team of scientists analyzed the chemical composition of several tiny grains retrieved from the comet 81P/Wild 2 by the NASA Stardust spacecraft and found that the samples are easily contaminated or modified during collection as well as during preparations to ready the samples for study. The results suggest that initial investigations of some of the grains after Stardust's return to Earth may have yielded a false picture of their chemical makeup, but also confirm that Stardust is capable of returning relatively pure comet samples.

The goal of the Stardust mission, which launched in February 1999, was to bring comet samples and interstellar dust particles back to Earth for analysis, mainly so that scientists could determine the mineral composition of the comet. Scientists also hoped the mission would deliver primitive organic matter that was present in the early stages of our solar system's formation and, importantly, that was “unprocessed” by exposure to heat and water. The first sample collection took place in 2004, with Stardust returning to Earth in 2006.

In this work, the researchers examined six grains that had been studied during Stardust’s preliminary examination phase as well as six untouched samples. They discovered that most had been compromised in some way during collection and suggest that the contamination was caused by contact with the material used to “catch” the samples, silica aerogel, a lightweight, airy solid derived from silica (i.e. silicon dioxide).

For example, dust particles are traveling extremely fast when they make contact with the aerogel (nearly four miles per second!), and the impact can heat the particle and the surrounding aerogel to temperatures exceeding 1200 degrees Celsius. In such extreme heat, some organic materials can vaporize and the samples and aerogel can melt. But other types of organic matter can survive it, and material within the grains, where it doesn't get so hot, may also be safe.

The researchers also assert that prepping the samples to be studied, such as by “ultramicrotoming” them into extremely thin slices, may introduce contaminants. Directly imaging the samples via transmission electron microscopy (TEM), as done here, is a key way to distinguish between true cometary material and contaminants, and assessing how much a sample may have been altered.

In addition to TEM, the researchers used x-ray absorption near-edge structure (XANES) spectroscopy, where analyzing how x-rays are absorbed by a sample yields information on its chemical composition; and secondary ion mass spectroscopy, in which researchers can learn what isotopes the sample contains by studying the “secondary” ions ejected from a sample after it is hit with a beam of primary ions. The x-ray studies were performed at three synchrotron facilities: NSLS (beamline X1A1), the Advanced Light Source at Lawrence Berkeley National Laboratory, and the Canadian Light Source.

Samples that are extremely rich in deuterium, an isotope of hydrogen with a nucleus consisting of a proton and a neutron (instead of just a proton), indicate organic matter of cometary origin, as that kind isotopic composition is not formed on Earth or elsewhere in the solar system. The other key factor was whether the samples showed evidence of aerogel contamination; those that didn't were probably not affected by the heating/mixing that could occur during capture. Therefore, samples that were both deuterium-rich and free of aerogel contamination likely represent pristine cometary organic matter.

The images and data collected from the 12 samples show several cases in which materials identified during the preliminary examination phase are clearly contaminants. Just three of the 12 appear to contain mostly unaltered cometary material.

From this, the researchers conclude that the “true” composition of organic matter on comet Wild 2 consists of two main types: “polyaromatic carbonyl-containing organic matter,” the type of organic material typically found in primitive meteorites and interplanetary dust particles; and “highly aromatic refractory organic matter,” also found in meteorites, which, in these cometary samples, consists mostly of hollow, spherical carbon-rich grains.

This research was performed by scientists from the Naval Research Laboratory, ESCG/NASA Johnson Space Center, the Carnegie Institute of Washington, Lawrence Berkeley National Laboratory, and Brookhaven National Laboratory. The results are published in the September 2011 issue of Meteoritics & Planetary Science.

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